US 7237594 B2
An injection-molding process for molding a metal alloy into a near net shape article that is characterized in that the processing temperature of the alloy at injection is approaching the liquidus, preferably having a maximum solids content of 5%, whereby a net-shape molded article can be produced that has a homogeneous, fine equi-axed structure without directional dendrites, and a minimum of entrapped porosity. Advantageously, the resulting solid article has optimal mechanical properties without the expected porosity and solidification shrinkage attributed to castings made from super-heated melts. Also disclosed is a metal-matrix composite including a metallic component, and also including a reinforcement component embedded in the metallic component, the metallic component and the reinforcement component molded, at a near-liquidus temperature of the metallic component, by a molding machine.
1. An injection-molding process for molding a metal alloy and a reinforcement component into a near net shape metal-matrix composite article including the following steps:
feeding the alloy and the reinforcement component into to an injection-molding apparatus having a heated barrel assembly;
transporting the alloy and the reinforcement component through a melt passageway in the barrel assembly with a screw feeder disposed therein and heating the alloy and the reinforcement component to a near-liquidus temperature of the alloy;
accumulating a volume of the alloy and the reinforcement component in an accumulation portion of the barrel assembly;
controlling the near-liquidus alloy temperature in the accumulation portion to maintain the alloy in a molten state having a maximum solids content of 3%; and
injecting the alloy and the reinforcement component to fill a mold and cast at the near-liquidus temperature into the near net shape metal-matrix composite article having a fine equi-axed structure substantially without coarse directional dendrites.
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This application is a continuation-in-part, claims priority to and the benefit of pending U.S. Ser. No. 10/985,879, filed on Nov. 10, 2004, entitled NEAR LIQUIDUS INJECTION MOLDING PROCESS.
This invention relates to an injection molding process for making near net-shape metal articles and in particular, relates to thin-walled metal articles made from metallic alloys, particularly light metals.
In conventional casting, the metal is superheated above its liquidus temperature (i.e. the liquidus being the temperature above which the alloy is completely liquid). A minimum superheat is required to ensure that the metal does not solidify prematurely, particularly when molding thin-walled molded articles. Superheating metals which are prone to oxidation has attendant process control challenges to provide and maintain an inert atmosphere.
Articles which are cast from superheated melts often are not sound in that shrinkage porosity and entrapped gases are not uncommon. In addition, their mechanical properties such as tensile strength, yield stress, and elongation suffer, and this is attributed to a microstructure characterized by coarse grains and dendrites.
These problems have been recognized and extensive work has been done to find other ways of processing metal alloys to improve the mechanical properties of cast articles. In particular, through the use of well known semi-solid metal processing techniques molded articles may be produced with much higher mechanical properties as a result of the generation of a favorable alloy microstructure and by reductions in alloy porosity. Moreover, semi-solid processing techniques provide further advantages in that the relatively low temperature of the alloy slurry provides for a longer useful life of the mold than the die-casting method (e.g. lower thermal shock, and reduced amount of liquid-metal corrosion caused by processing fully molten metals), and improved molding accuracy of the molded article. Common semi-solid processing techniques include semi-solid injection molding, rheocasting, and thixoforming.
Semi-solid injection molding (SSIM) is a metals-processing technique that utilizes a single machine for injecting alloys in a semi-solid state into a mold to form an article of nearly net (final) shape. SSIM involves the steps of partial melting of an alloy material by the controlled heating thereof to a temperature between the liquidus and the solidus (i.e. the solidus being the temperature below which the alloy is completely solid) and then injecting the slurry into a molding cavity of an injection mold. SSIM avoids the formation of dendritic features in the microstructure of the molded alloy, which are generally believed to be detrimental to the mechanical properties of the molded article. The structure and steps of SSIM are described in more detail with reference to the description of the preferred embodiment of the present invention provided hereinafter and with reference to U.S. Pat. No. 6,494,703, the disclosure of which is herein incorporated by reference.
By contrast, rheocasting refers to a process of manufacturing billets or molded articles through casting or forging semi-solid metallic slurries having a predetermined viscosity. In conventional rheocasting, molten alloy is cooled from a superheated state and stirred at temperatures below the liquidus to convert dendritic structures into spherical particles suitable for rheocasting, for example, by mechanical stirring, electromagnetic stirring, gas bubbling, low-frequency, high-frequency, or electromagnetic wave vibration, electrical shock agitation, etc.
Thixocasting refers to a process involving reheating billets manufactured through rheocasting back into a metal slurry and casting or forging it to manufacture final molded articles.
For instance, U.S. Pat. No. 5,901,778 describes an improved rheocasting method and extruder apparatus for producing a semi-solid metal alloy slurry having a solids content between 1 and 50% that is characterized by structure and steps whereby molten metallic alloy material is introduced into an agitation chamber, that is heated about 100 degree C higher than a liquidus temperature of the molten metallic material, wherein the alloy is cooled and agitated by a cooled screw-shaped stirring rod, having a temperature below a temperature of the semi-solid, to produce the semi-solid slurry.
U.S. patent application Ser. No. 2004/0173337 describes an improved rheocasting method and apparatus for producing a non-dendritic, semi-solid metal alloy slurry having a solids content of about 10% to about 65% that is characterized by structure and steps whereby problems associated with accumulation and removal of metal from surfaces of the apparatus contacting the slurry are reduced or eliminated.
U.S. patent application Ser. No. 2004/0055726 describes a rheocasting method and apparatus for die casting molded articles that is characterized by structure and steps for applying an electromagnetic field to stir a molten metal as it is being loaded into a slurry forming portion of a shot sleeve whereby the slurry is stirred until cooled below its liquidus temperature prior to its transfer to a casting portion of the shot sleeve. Preferably, the stirring is maintained until the slurry achieves a solid fraction in the range of 0.1 to 40%, alternatively the slurry is stirred until the solid fraction is in the range of 10 to 70%. Related U.S. patent applications Ser. Nos. 2004/0055727, 2004/0055734, and 2004/0055735 describe similar structure and steps for manufacturing billets for thixocasting, manufacturing metallic materials for rheocasting or thixoforming, and for manufacturing a semi-solid metallic slurry, respectively.
U.S. Pat. No. 6,311,759 describes a process for producing a feedstock billet material that is characterized in that it is produced from a melt at substantially its liquidus temperature whereby a microstructure of the feedstock is rendered especially suitable for subsequent thixocasting in the semi-solid range of 60 to 80% primary solids. This patent is significant in that it recognizes that metal alloys cast from at a near liquidus temperature will result in a favorable grain structure characterized by primary grains that are equi-axed and globular with no dendrites.
The process of SSIM is however generally preferred as it provides for several important advantages relative to the other semi-solid processing techniques. The benefits of SSIM include an increased design flexibility of the final article, a low-porosity article as molded (i.e., without subsequent heat treatment), a uniform article microstructure, and articles with mechanical and surface-finish properties that are superior to those made by conventional casting. Also, because the entire process takes place in one machine and in an ambient environment of inert gas (e.g., argon), alloy evaporation and oxidation can be nearly eliminated. The SSIM process also provides for energy savings in that it does not require the heating of the alloy above its liquidus temperature.
Although a 5-60% solids content is generally understood to be the working range for SSIM, it is also generally understood that practical guidelines recommend a range of 5-10% solids for injection molding thin-walled articles (i.e., articles with fine features) and 25-30% for articles with thick walls. The foregoing is described in U.S. Pat. No. 5,040,589.
Notwithstanding the foregoing, a recently published discovery by the inventor of the present invention has shown that the range of percentage of solids in SSIM processing can be advantageously extended into an ultra-high solids range between 60 and 85%. The foregoing ultra-high solids process is fully described in commonly assigned U.S. patent application Ser. No. 2003/0230392.
The lower limit of 5% solids fraction has been sustained by those skilled in the art because of a belief that to lower the solids fraction any further would obviate any advantages achieved by semi-sold processing. In particular, with a low or non-existent solids content, the fluidity of the alloy is expected to increase, resulting in an increase in turbulence in the flow front thereof as the molding cavity is being filled, and thereby increasing the likelihood of porosity and entrapped gases in the final article.
Notwithstanding the foregoing, it is known to configure structure and steps for SSIM processing with a percentage of solids as low as 2% under certain conditions.
For instance, U.S. Pat. No. 5,979,535 describes a method for injection molding a molded article having both lower and higher solid fraction portions therein, the method characterized in that structure and steps are provided for establishing a temperature distribution in the semi-molten slurry in the direction of injection, by the controlled heating thereof in an extruder cylinder, whereby the slurry contemporaneously includes a low and a high solids fraction portions for sequential injection into the molding cavity. In a cited example, an orifice holder is molded in which a high strength head portion is formed from a melt portion having about 2% solids whereas a more accurately molded threaded portion is formed from a melt portion having about 10% solids.
However, the molding of thin-walled molded articles, particularly those having a thickness below 2 mm, using SSIM at typical low levels of solids fraction (i.e. 5%) can be problematic because of premature alloy solidification that results from the reduced fluidity of the alloy metal, relative to die casting, and because of the high thermal conductivity of typical molding alloys (e.g. Magnesium alloy AZ91D).
U.S. Pat. No. 6,619,370 is directed at solving the problems of molding thin-walled molded articles using SSIM. In particular, structure and steps are provided for increasing the fluidity of the semi-molten melt and for providing increased degassing of the molding cavity. It is stated therein that the solid fraction of the semi-molten metal slurry must be set within a range exceeding 3% and below 40% to avoid excessive warping of the thin-walled molded article.
However, it remains a challenge to produce thin-walled molded articles using SSIM without resort to significant overheating of the alloy above the liquidus temperature and the resulting reduction in mechanical properties.
Accordingly, an advantage of the present invention is that an injection molding process is provided for producing thin-walled metal articles with improved structural integrity and superior mechanical properties relative to those produced by traditional casting methods.
In accordance with an aspect of the present invention, an injection-molding process is provided for molding a metal alloy into a near net shape article in which the processing temperature of the alloy is approaching its liquidus, preferably having a maximum solids content of 5%, whereby a net-shape molded article can be produced that has a homogeneous, fine equi-axed structure without directional dendrites, and a minimum of entrapped porosity.
Advantageously, the resulting solid article has optimal mechanical properties without the expected porosity and solidification shrinkage attributed to castings made from super-heated melts.
In accordance with another aspect of the present invention, an injection-molding process is provided for molding a metal alloy into a near net shape article in which the processing temperature of the alloy is approaching its liquidus, preferably having a maximum solids content of 2%, whereby a net-shape molded article can be produced that has a homogeneous, fine equi-axed structure without directional dendrites, and a minimum of entrapped porosity.
In accordance with a preferred embodiment of the present invention the magnesium alloy AZ91D is to be processed at a temperature range of within 2° C., preferably below, its liquidus temperature. The target liquidus temperature itself may need to be ascertained by trial and error to adjust for composition changes in the feed alloy, and changing heat transfer conditions between the barrel and the melt. For a nominal composition of the Az91D alloy, the alloy is to be heated in the barrel to a processing temperature approaching 595° C.
In accordance with an alternative embodiment of the present invention the magnesium alloy AM60B is to be processed at a temperature range of within 1° C., preferably below, its liquidus temperature. The target liquidus temperature itself may need to be ascertained by trial and error to adjust for composition changes in the feed alloy, and changing heat transfer conditions between the barrel and the melt. For a nominal composition of the AM60B alloy, the alloy is to be heated in the barrel to a processing temperature approaching 615° C.
The invention finds application to the fabrication of thin-walled articles such as casings for laptop computers, video recorders and cell phones made from light metal alloys. Magnesium based alloys are of particular interest because of their superior strength to weight ratio, stiffness, electrical conductivity, heat dissipation and absorption of vibrations.
According to another aspect of the present invention, there is provided a metal-matrix composite, including a metallic component, and also including a reinforcement component embedded in the metallic component, the metallic component and the reinforcement component molded, at a near-liquidus temperature of the metallic component, by a molding machine.
According to yet another aspect of the present invention, there is provided a molded article, including a metallic component molded, at a near-liquidus temperature of the metallic component.
In order to better understand the invention, a preferred embodiment is described below with reference to the accompanying drawings, in which:
Solid chips of alloy material are supplied into the melt passageway of the barrel assembly through a feeder apparatus 18. The alloy chips may be produced by any known technique, including mechanical chipping or rapidly solidified granules. The size of the chips is approximately 1-3 mm. A rotary drive portion 20 turns a retractable screw portion 22 that is arranged in the melt passageway of the barrel portion 12 to transport the alloy material therealong.
Experiments were conducted using two commercial die cast alloys Az91D and AM60B whose nominal compositions are shown in Table 1. Another suitable alloy is AJ52 (Mg—5Al—1.5Sr) as described in U.S. Pat. No. 6,808,679 that has a nominal liquidus temperature of 616° C. It should be understood, however, that the present invention is not limited to the injection molding of magnesium alloys but is also applicable to injection molding of other alloys, including Al alloys and other alloys such as lead based alloys, zinc based alloys, and bismuth based alloys.
In accordance with a preferred near liquidus molding process of the present invention, the heaters 14 are controlled by microprocessors (not shown) programmed to establish a precise temperature distribution within the barrel portion 12 that heats the alloy in the melt passageway of the barrel assembly to a temperature approaching its liquidus so that the solids fraction is preferably 0% but not over 5%.
Motion of the screw portion 22 acts to mix the alloy as it is being melted and to convey the melt past a non-return valve 26, mounted at a distal end of the screw, for accumulation of the melt in a forward portion of the melt passageway, a so-called “accumulation portion” of the barrel. The non-return valve 26 prevents the melt from squeezing backwards into the barrel portion 12 during injection.
The internal portions of the apparatus 10 are kept in an inert gas surrounding to prevent oxidation of the alloy material. An example of a suitable inert gas is argon. The inert gas is introduced via the feeder 18 into the apparatus 10, which prevents the back-flow of air. Additionally, a plug of solid alloy, is formed in the nozzle portion 16 after injection. The plug is expelled when the next shot of alloy is injected and is captured in a sprue post portion of the mold 24.
The rotary drive portion 20 is controlled by a microprocessor (not shown) programmed to reproducibly transport each shot of alloy material through the barrel portion 12 at a set velocity, so that the residence time of each shot in the different temperature zones of the barrel portion 12 is precisely controlled, thus reproducibly minimizing the solids content of each shot to ensure that it does not exceed a 5% solids fraction.
Experiments were conducted in accordance with the invention to apply the injection molding technique for the net-shape forming of Mg—9Al—1Zn and Mg—6Al particulates, after preheating to near-liquidus ranges, and assess the microstructural and tensile characteristics of the solidified alloys. As a comparison base, the same alloy grades were used after processing from a superheated liquid by conventional die-casting.
During injection molding, the feedstock, in the form of mechanically comminuted chips, was processed in a Husky TXM500-M70 system with a clamp force of 500 tons and equipped with a tensile bar mold. The total weight of the four cavity shot was 250.3 g, including 143.7 g of sprue with runners and 35 g of overflows. Upon accumulating the required shot size in front of the non-return valve, the screw was accelerated forward to 2.2 m/s, injecting the alloy through the sprue and gates with an opening area of 64.8 mm2 into the mold cavity, preheated to 200° C. After the mold 24 is filled with the slurry, the slurry may undergo a final densification, in which pressure is applied to the slurry for a short period of time, typically less than 10 ms, before the molded article is removed from the mold 24. The final densification is believed to reduce the internal porosity of the molded article.
The alloys with nominally the same chemistries were also processed into tensile bars using a Bueler Evolution 420D high-pressure die casting machine at Hydro Research Park, Porsgrunn, Norway. The die was preheated to 200° C. and the temperatures of AZ91D and AM60B melts were 670° C. and 680° C., respectively.
Tensile testing was conducted according to ASTM B557 using cylindrical samples with a reduced section diameter of 6.3 mm for molding and 5.9 mm for die casting, and a gauge length of 50.8 mm. Measurements were performed using an Instron 4476 machine equipped in an extensometer at a crosshead speed of 0.5 mm/min. Tensile curves were analyzed to assess the ultimate tensile strength, yield strength and elongation. The chemical compositions were determined with inductive coupled plasma spectrometry according to ASTM E1097-97 modified and E1479-99 specifications. Cross sections for optical microscopy observations were prepared by polishing down to 0.05 μm de-agglomerated alumina powder. To reveal microstructure, surfaces were etched with 1% nital. Moreover, an etching was used to show differences in crystallographic orientations of individual grains. The stereological parameters of selected microstructures were measured using the quantitative image analyzer. The structural details were imaged with scanning electron microscopy (SEM) and the microchemistry was measured with an X-ray microanalyzer (EDAX). X-ray diffractometry with CuKα radiation was applied for the phase and crystallographic characterizations of materials.
Melting Differences of AZ91 and AM60 Alloys
The Mg—rich portion of the binary Mg—Al diagram with the marked locations of examined alloys and processing temperatures is shown in
The AZ91D and AM60B alloys exhibit approximately 20° C. difference in their liquidus temperatures of nominally 595° C. and 615° C., respectively. For both chemistries, the specific solid content fs can be calculated according to Scheil's equation:
where Tm is the melting temperature of pure metal, TL is the liquidus temperature of the alloy and Ko is the equilibrium distribution coefficient. The results are presented in the form of a graph in
The comparative graph of tensile strength plotted, versus corresponding elongations for both alloys and processing techniques, is shown in
There was a scatter of experimental data points for each alloy composition and processing method, with a general tendency of the higher strength corresponding to the higher elongation (
Alloy's Structural Integrity
As factors affecting structural integrity of the alloy, only those defects which are inherent to the given processing method are discussed here. The defects which are associated with incorrect injection and thermal settings or the specific part geometry, are not considered. Due to the very simple geometry of the selected mold (die), virtually no macro porosity occurred in the 5.9 and 6.3 mm sections of tensile bars (
The predominant or exclusive component of microstructures generated during molding in a near-liquidus range was the solidification product of the liquid fraction (
In addition to the matrix, a negligible fraction of the primary solid phase was present (
The microstructures produced from a superheated liquid by die-casting are shown in
An etching technique was used as a method for the qualitative assessment of differences in crystallographic orientation between microstructural constituents. The color distribution within the microstructure, obtained by near liquidus molding, revealed that there is no dominant preferred orientation (
The alloys die cast from the superheated liquid range showed large dendrites, suggesting that all features within a dendrite had the same or very similar crystallographic orientation. Some of them had the morphology of primary dendrites, formed prior to injection into a mold cavity. The etching showed that many features portrayed on conventional micrographs as individual grains, were in fact a part of the large multi-grain conglomerates (e.g.
The X-ray diffraction provided information about the crystallography of phases, their contents and an estimation of the preferred orientation. The AZ91D alloy, molded from the near liquidus range, contained the α—Mg and intermetallic phase of Mg17Al12 (
There was a significant difference in the morphology of the de-cohesion surface between the near-liquidus molded and the superheated liquid die cast structures. The typical cross-sectional view of an AZ91D tensile bar after near-liquidus molding is shown in
The dendritic morphologies present within the alloy, processed from the superheated liquid, exerted a profound influence on the fracture mechanism (
The experiments conducted show that the injection molding of magnesium alloys, preheated to tight temperatures around the liquidus value, diminishes some disadvantages typical for the casting of superheated melts. Negligible porosity (
The operating temperatures at around 70-100° C. lower than the die cast alloys also brings advantages expressed by energy savings, reduced deterioration of machine/mold components and reduced alloy losses by evaporation and oxidation. Since injection molding relies on the barrel sealing concept using a thermal plug, it does not allow for substantial overheating of the molten alloy. Therefore, as a processing which utilizes a superheated melt, die-casting was selected here. Both the hot and cold chamber die castings start from a superheated liquid and suffer from the disadvantage that it is difficult to produce fully sound components. A superheating is required to compensate for the heat loss during transfer to and delay time in the hot sleeve. There are a number of key differences between die-casting and injection molding at all stages of processing and the alloy's temperature is only one of them. This should be kept in mind while comparing results obtained by both techniques.
In addition to the component's integrity, the processing temperature exerts an effect on the alloy microstructure (
Another factor, potentially affecting the solidification process of a molded alloy, is the agitation exerted by the reciprocating screw during conveyance along the barrel and high injection speed during mold filling. In fact, it is difficult to separate those two contributions. Turbulence introduced by high intensity shear affects destabilization of diffusion boundary layer and also prevents solute build up ahead of the solid-liquid interface and thus suppresses dendritic growth due to compositional undercooling. As seen in
For semi-solid processing, the room temperature microstructure allows us to reproduce a thermal history of the alloy. While exploring the near-liquidus temperatures, the features which provide the link to the processing parameters, are less distinct. For sub-liquidus molding, the alloy's temperature may be estimated based on measurements of the unmelted solid fraction. A lack of entrapped liquid does not allow distinguishing between rheo- and thixo-routes, meaning that it is not an indication whether the liquidus temperature was achieved from the solid or liquid direction (
While considering the beneficial changes of mechanical properties after semi-solid processing, two factors are frequently mixed: (i) an improvement caused by a reduction in porosity and (ii) a change due to a modification of the microstructure. It is clear that the high integrity structures, generated after near-liquidus molding, take advantage of the first factor. Experiments conducted here allow assessing the influence of the structure-related factor. A variation in tensile properties of both molded alloys, shown in
where d represents grain size. The maximum strength of 124 MPa in formula (2) for fs equal 0 is significantly lower than values reported in
When comparing the AZ91D and AM60B grades, the major difference is the higher elongation of the latter. It is generally accepted with the quantitative evidence published that the first alloying approach for better toughness is to reduce the volume fraction of the Mg17Al12 intermetallic phase: the content of Mg17Al12 was in the range of 2-7% for AM60 grade and from 5 to 16% for AZ91D. Thus, the higher elongation of AM60B in
It is generally accepted that semi-solid processing provides properties which are superior over those obtained after conventional casting. While the foregoing can be shown for Al alloys, for Mg—Al and Mg—Al—Zn alloys an increased solid content has shown a reduction in both strength and ductility. The metallurgical characteristics gathered here and in previous research as shown in
It is also expected that similar results will be obtained with near-liquidus molding of other alloys suitable for injection molding, as will be appreciated by those skilled in the art.
The injection molding system allows implementing a concept of near liquidus processing which requires a tight control of the alloy's temperature such that the alloy is maintained at a near-liquidus temperature, as close to the molding cavity as possible. The injection mold 24 is preferably configured to include at least one temperature controlled melt conduit such as a hot sprue or a hot runner to convey the melt to the gate during injection and maintain it at processing temperatures between injection cycles. A suitable system is described in Applicant's co-pending U.S. patent Office application Ser. No. 10/846,516, the disclosure of which is herein incorporated by reference. By using such a system, the flow distance between the molten alloy with a controlled temperature and the mold gates is reduced, thus minimizing a drop in temperature. Preventing heat losses has a particular meaning for magnesium alloys, known for their low thermal capacity and tendency to quick solidification, which disrupts the complete filling of the mold.
The molding of Mg—9Al—1Zn and Mg—6Al alloys, after preheating to a narrow temperature range around the liquidus level, leads to the formation of high-integrity structures. Shrinkage porosity, unavoidably present after conventional casting, which utilizes superheated melts, is minimized to negligible level.
The matrix of near-liquidus molded Mg—9Al—1Zn and Mg—6Al alloys is macroscopically homogeneous and consists of fine equi-axed structures of α—Mg with a typical size of 20 mm and no coarse directional dendrites which would result from pre-eutectic solidification. The α—Mg grains are surrounded by mostly discontinuous precipitates of the Mg17Al12 intermetallic phase with a slightly higher content than after casting from superheated melts. The primary solid is either completely absent or present in negligible amounts, not exceeding 5% of volume fraction. The solid particles do not contain any entrapped liquid and represent a morphology from spheroids to degenerated rosettes, depending on the thermal profile along the alloy's flow path within the system.
The near-liquidus molded Mg—9Al—1Zn and Mg—6Al alloys exhibit a superior combination of strength and elongation than their counterparts produced from the superheated liquid and by the semi-solid route. The tensile properties benefit from high structural integrity and fine microstructure.
A metal-matrix composite is a combination of a metallic component with a reinforcement component. The reinforcement component is usually non-metallic and is commonly a ceramic or other material such as (for example): continuous fibers such as boron, silicon carbide, graphite or alumina; wires including tungsten, beryllium, titanium and molybdenum; and/or discontinuous materials such as fibers, whiskers and particulates. The metal component provides a compliant support for the reinforcement component. The reinforcement component is embedded into the metal component. The reinforcement component does not always serve a purely structural task (reinforcing the metal component), but is also used to change physical properties such as wear resistance, friction coefficient, thermal conductivity, stiffness, strength, heat resistance, etc. The reinforcement component can be either continuous or discontinuous. A discontinuous metal-matrix composite is isotropic and can be worked with standard metalworking techniques. A continuous reinforcement component uses monofilament wires or fibers such as carbon fiber or silicon carbide. Because the fibers are embedded into the metal component in a certain direction, the result is an anisotropic structure in which the alignment of the material affects its strength. One of the first metal-matrix composites used boron filament as the reinforcement component. The discontinuous reinforcement component uses “whiskers”, short fibers, or particles.
The metal-matrix composite is produced by means of processes other than conventional metal alloying. The metal-matrix composite is often produced by combining two pre-existing constituents (such as, a metal and a ceramic fiber). Processes commonly used include powder metallurgy, diffusion bonding, liquid phase sintering, squeeze-infiltration and stir-casting.
Alternatively, typical high-reactivity of metals at processing temperatures can be exploited to form the reinforcement component and/or the metal-matrix composite in situ (that is, by chemical reaction within a precursor of the metal-matrix composite).
A metal-matrix composite (including a metallic component and a reinforcement component embedded in the metallic component) was molded at a near-liquidus temperature of the metallic component by a molding process of an injection molding machine. The injection molding machine was a Husky™ Thixo 5 injection-style molding machine. Generally the method involved maintaining or controlling a temperature of a slurry of the metal-matrix-composite (which was located in at least a part of the molding machine, preferably located in a head portion of the molding machine) within a temperature range near to (relative to and/or there around) the liquidus temperature of the metallic component so that the slurry of the metal matrix composite had a solid content that ranged from about 0% to about 5%. It will be appreciated that the temperature range will vary depending on the alloy used. A metal matrix composite that was made by this method included a metallic component molded by a molding machine, that was configured to control a temperature of the slurry within a temperature range near the liquidus temperature of the metallic component, and the slurry had a solid content ranging from about 0% to about 5%.
By way of example, for a slurry of a metal-matrix composite that included a metallic component having an alloy of Mg (specifically: AZ91), in which the liquidus temperature of the AZ19 alloy was about 695 degrees Celsius, the temperature of the slurry was held, in at least a part of the molding machine) within a temperature range that extended from about 695 degrees Celsius to about 693 degrees Celsius (that is: about 695 degrees Celsius minus about 2 degrees Celsius). A molded metal matrix composite having the alloy AZ19 of Mg had a solid content that ranged from about 0% to about 5%. It will be appreciated that the temperature range of other metal-matrix composites will be different, and the temperature range will depend on the type of alloy included in the metallic component of the metal-matrix composite.
In a preferred embodiment, the metallic component included a magnesium (Mg) alloy, and the reinforcement component included either finely-granulated particles of silicon carbide (SiC). In an alternative embodiment, the metallic component includes a magnesium-based alloy and/or an aluminum-based alloy and/or a zinc-based alloy and any combination and permutation thereof. The magnesium alloy was AZ91D having a low solid content.
The specimen molded by the molding machine was a tensile bar. The tensile bar is an injection-molded specimen of specified dimensions, and the specimen is used to determine tensile properties of a material included in the specimen.
The preferred method included the following steps or operations: A mold defining four molding cavities was preheated to 200 degrees Celsius (° C.). Chips of magnesium and a predetermined volume of SiC particles were introduced into a molding machine hopper that was coupled to the molding machine. The silicon carbide particles (with different sizes) were added in different rates and volumes. The nature (either thixo and/or rheo) of the metal-matrix composite was not controlled in a barrel of the molding machine. During flow within a barrel of the molding machine, SiC particles were mixed with the magnesium alloy that was heated to a semisolid state. The molding machine was arranged to accumulate a shot of the metal-matrix composite having a predetermined shot size. Preferably, the metallic component included a metallic-alloy slurry that had a “controlled” amount of solid content while processed in the barrel (it will be appreciated that this condition is not a necessary condition).
The preferred method also included the following steps or operations: A total weight of the shot was computed to be 250.3 grams (g), which included 143.7 g of sprue with runners and 35 g of overflows. The shot was accumulated in front of a non-return valve. A processing screw was accelerated forwardly to approximately 2 metres per second (m/s), and as a result the shot was injected through the sprue and the gates and then into the four mold cavities. Further mixing of the SiC particles took place during filling of the mold cavities. It is believed that the SiC particles were sufficiently homogeneously distributed within the molded tensile bar. The sprue and the gates defined passageways therein has a cross sectional area of 65 square millimeters (mm2). The barrel of the molding machine that contained the screw had a diameter of 70 mm and a length of approximately of 2 m (metres). A thermal profile of the barrel was controlled by electric-resistance heaters placed onto the barrel, and the heaters were grouped into heating zones. The thermal profile of the barrel was arranged so that the molded metal matrix composite included the metallic component that had a fraction of an un-melted phase from about 0% to about 5%.
In an alternative, the reinforcement component was selected to be chemically reactive, at least in part, with the metallic component. In another alternative, the reinforcement component was selected to be chemically non-reactive with the metallic component.
In an alternative, the reinforcement component included a metallic alloy. In another alternative, the reinforcement component included a non-metallic component. In yet another alternative, the reinforcement component included a powder. In yet another alternative, the reinforcement component included boron nitride (BN).
The following is a discussion of the metallographical assessment of a metal-matrix composite molded at near-liquidus temperature. A technical result of the embodiment is that the SiC particles are substantially uniformly distributed within the metal-matrix composite.
According to another embodiment, a molded article includes a metallic component molded, at a near-liquidus temperature of the metallic component. Preferably, while the metallic component existed in a slurry state, the metallic component had a solid content up to 5%. Preferably, the metallic component molded was molded by a molding machine. Preferably, the metallic component molded was molded by a molding machine, and the molding machine included an injection molding machine.
While the present invention has been described with respect to what is presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.